I never thought I'd see a show that I thought could be competition for The Wire when it comes to best show, but for me West Wing comes really, really close.
I'm watching it for the first time right now, I'm just starting season 4. It's incredibly tightly written, it fleshes out characterization without being obvious about it, it adds just enough humor to what's a very serious concept, it straddles the line between drama and melodrama and rarely falls to the wrong side of it, it's never trite, and it has something to say about the world beyond just itself. Like The Wire, it's almost a TV novel-- there are no monster of the week type episodes that just serve as placeholders, the storyline spirals out around itself with the characters growing naturalistically.
And most importantly, it treats its audience as smart and never dumbs down its content, and its research is impeccable. I think sometimes it's too idealistic and not realistic enough, but I think that, too, is part of its core concept. It's saying "this show is operating in the real political world, here is how a presidency *could* work if Americans participated more and demanded more of its government than someone who's "just folks" and doesn't know how to lead". IMO for a network show, it's impeccable.
Past that, I can think of a number of shows I think are good and have a ton of potential, but nothing I'd put up as truly great. I'd love to say Doctor Who, but it has too many throw-away episodes and way too much melodrama (Ten's exit was protracted, self-indulgent, and almost painfully embarrassing, IMO).